The compressor section of a gas turbine engine includes a diffuser downstream of the compressor. The function of the diffuser is to reduce the velocity of the compressed air and simultaneously increase the static pressure, thereby preparing the air for entry into the combustor at a lower velocity. Presenting high-pressure and low-velocity air to the combustor section is essential for proper fuel mixing and efficient combustion.
A centrifugal compressor impeller draws air axially, and rotation of the impeller increases the velocity of the air flow as the input air is directed over impeller vanes to flow in a radially outward direction under centrifugal forces. In order to redirect the radial flow of air exiting the impeller to an annular axial flow for presentation to the combustor, a diffuser assembly is provided which redirects the flow as it also reduces the velocity and increases static pressure of the air flow.
A conventional diffuser assembly of this type, sometimes known as a fishtail diffuser, generally comprises a machined ring which surrounds the periphery of the impeller for capturing the radial flow of air and redirecting it through generally tangential orifices into an array of diffuser tubes. The orifices in the diffuser ring are circumferentially spaced apart, each one being intersected by two adjacent bores in an asymmetrical configuration. The diffuser tubes are generally brazed or mechanically connected to the ring and have an expanding cross-section rearwardly.
In general, the design of diffusers requires a compromise between the desired aerodynamic properties and the practical limits of manufacturing procedures. For example, the orifices in the impeller surrounding ring are typically cylindrical bores or conical bores due to the limitations of economical drilling procedures. To provide elliptical holes for example, would involve prohibitively high costs in preparation and quality control.
Engine performance is directly affected by the quality of the tangential diffuser bores. For good performance, a very accurate diameter and true position of these bores, a sharp edge of the bore intersection area and a very good surface finish of these bores are all required. This makes the diffuser one of the most costly and difficult parts of the gas turbine engine to manufacture.
The manufacturing process for the diffuser typically includes both roughing and finishing operations on its various surfaces. It is common practice to complete the roughing operation for all surfaces before beginning the finishing operation. This is done for convenience of changing tools, etc., and more importantly to prevent damage to the finished surfaces by completing the roughing first. Conventionally, diffuser bores in a diffuser ring are machined with a gun drilling machine which performs the roughing process for all bores in the diffuser ring, and then the finishing process is performed with a cylindrical and/or taper reamer.
Because of the configuration of the intersecting bores in a roughed-out diffuser, the finishing tool is always between the two intersections of the adjacent bores when finishing the bores. The two intersections of adjacent bores are not symmetrical, and therefore, the radial cutting force on the finishing tool is unbalanced, creating undesirable tool deflection, which results in poor quality of both position and diameter.
Furthermore, the unbalanced radial cutting force and the tool deflection inhibit the use of carbide tools which are adapted for high speed cutting but are too brittle to handle tool deflections normal in this type of operation. Thus, productivity of the diffuser bore machining process is limited. The conventional process also cannot provide a superior quality of surface finishing of the diffuser bores because the asymmetrical intersections of each diffuser bore limits the use of super-finishing tools such as burnishing tools.
Therefore, an improved process for machining the bores in the diffuser ring with better quality control and better productivity is desired.